1. Field of the Invention
The present invention relates to communications, and, in particular, to spread-spectrum wireless communications systems.
2. Description of the Related Art
The IS-95 standard, an interim standard published by the Telecommunications Industry Association, is an existing wireless communications standard that is based on spread spectrum communication techniques, also known as code division multiple access (CDMA) techniques. As is known in the art, CDMA techniques employ channels distinguished by different spreading codes. By combining each signal with the spreading code for the channel, each signal is spread over a much wider frequency band than the frequency band occupied by the signal prior to combining with the spreading code. This differs from traditional time division multiple access in which each channel transmits during a unique time frame and frequency division multiple access systems in which each channel is designated a unique portion of an available frequency band and/or modulates a unique carrier.
FIG. 1 is a block diagram showing a typical wireless IS-95 network 100. Network 100 includes a group of remote users 112-115 generally in communication with base stations 109-110 through an air interface. Base stations 109-110 are, in turn, connected to a land line network 102 through a switching center 104, which tracks the positions of remote users 112-115 in the network and allocates capacity of base stations 109-110 to remote users 112-115.
In FIG. 2, there is shown a block diagram of a transmit portion, or forward link, of a base station 109-110 of the network 100. The forward link in a communications network is the communication path through a CDMA communication channel of the air interface from a base station 110, for example, to one or more of remote users 112-115 (e.g., wireless telephones). The reverse link is defined for each remote user and is the communication path from one of the remote users 112-115 to the base station 110. For the forward link, digital signal processing block 202 performs processing of voice, voiceband data, or digital data signals from the land line network 102. Radio Frequency (RF) modulation section 204 typically receives the processed signals from digital signal processing block 202, and modulates an RF carrier signal with the processed signals in multiplier 208. The optional D/A converter 206 converts a digital bit stream of the processed signals to analog signals used to amplitude or frequency modulate the RF carrier signal. The D/A converter 206 is shown as an option since, in alternative systems, digital bit values of the processed signals may be used to directly modulate the phase of the RF carrier signal. The modulated RF carrier signal is typically a low-power signal, which is then amplified to a high power signal in RF amplifier 210. The high power signal is filtered in transmit filter 212, and provided to air interface by antenna 214.
FIG. 2 illustrates a single modulation path for a processed IS-95 signal modulating a single RF carrier signal and occupying, for example, a 1.25-MHz bandwidth. However, as is known in the art, multiple processed IS-95 signals may be transmitted in different frequency bands, each having a 1.25-Mhz bandwidth. An IS-95 transmit portion having several IS-95 signals modulating M carriers and transmitted in M different frequency bands is shown in FIG. 3.
Referring now to FIG. 3, there is shown a block diagram of a transmit portion of a base station (e.g. 110) of IS-95 wireless network 100. Base station 110 of wireless network 100 comprises M IS-95 signal generators 302 (where M is an integer greater than 0), combiner 304, and RF circuitry 306 and antenna 308. Each signal generator 302 receives low-rate (narrowband) data streams for up to 64 different users and processes that low-rate data to generate a communications signal conforming to the IS-95 standard. Each signal generator 302 of a base station 110 in network 100 generates an IS-95 signal at a different carrier frequency. The signals from the different signal generators are combined by combiner 304, which may typically be an analog RF combiner. The combined signal is processed by high-power RF circuitry 306 for transmission by antenna 308 to any number of the wireless unit remote users 112-115.
According to the IS-95 standard, the narrowband data stream for each user is multiplied by a particular code sequence and then modulated at a particular carrier frequency. For a given signal generator 302, the narrowband data stream for each user is encoded with a different code sequence, but modulated at the same carrier frequency. The effect of modulating the narrowband data for multiple users at the same carrier frequency is to spread all of the narrowband data for each user over the entire carrier-frequency band. In order to ensure that the modulated signals for different users do not interfere with one another, the code sequences are selected to ensure that the modulated signal for each user is orthogonal to the modulated signals for all other users in the same carrier-frequency band.
The IS-95 standard employs an RF signal for a carrier-frequency band that has a 1.25-MHz bandwidth, and which contains the encoded samples of several (up to 64) user conversations (voice or data sessions). Each user conversation comprises a baseband user signal of up to 9.6 Kbps, or possibly 14.4 Kbps, that is spread in bandwidth by a 1.228-MHz direct sequence digital encoding signal. The spreading rate, also known as the chip-rate, is therefore 1.228-MHz in the IS-95 standard. The encoding is achieved by using, for each user conversation, one of a set of 64 orthogonal Walsh codes, also known as Walsh functions or Walsh sequences. The Walsh codes of a given set are orthogonal in that the receiver reproduces the original user signal only if the received signal is demodulated with the same Walsh code used at the transmitter. Otherwise, uncorrelated noise is produced in the receiver. The digital signals of each user can simply be added together before being applied to the modulation part of the RF subsystem, as shown in FIG. 3.
Referring now to FIG. 4, there is shown a block diagram of a portion of each signal generator 302 of FIG. 3 of base station 110 of wireless network 100. According to the IS-95 standard, each signal generator 302 is capable of supporting low-rate (narrowband) data streams for up to 64 different users using a single carrier frequency. Each user is assigned a different one of 64 orthogonal IS-95 forward-link Walsh codes. FIG. 4 shows the processing performed on the data stream for one of the users supported by an exemplary implementation of signal generator 302. That is, blocks 402, 404, 406, 408, 410 and 412 shown in FIG. 4 would be repeated within signal generator 302 for each user with its own data.
In particular, for a single user data stream, convolutional encoder 402 provides a degree of error protection by applying convolutional encoding to the user""s data stream to generate encoded signals. Block interleaver 404 applies block interleaving to the encoded signals to generate interleaved signals. Block interleaver 404 provides further error protection by scrambling data in time. In a parallel path, long pseudo-noise (PN) code generator 406 generates code sequences that are then decimated by an integer value in decimator 408 to reduce the length of the sequence so as to prevent identification of the sequence. The sequences provided by the long PN code generator 406 and decimator 408 perform encryption to provide a degree of security to the communications process. Multiplier 410 combines the interleaved signals from block interleaver 404 with the decimated code signals from decimator 408.
The resulting signals from multiplier 410 are then combined with one of the 64 different Walsh sequences WN by Walsh-code multiplier 412. Multiplying signals by a unique Walsh sequence WN makes the resulting signals orthogonal to (and therefore non-interfering with) the signals for all of the other users of signal generator 302, each of which is multiplied by a different Walsh sequence.
For multiple users, the signals generated by each user""s Walsh-code multiplier 412 are summed in summer 413, and then processed along two parallel paths. In the first path, multiplier 414 combines the summed signals from Walsh-code multipliers 412 with the signal PI(t) and the signals from multiplier 414 are then combined by multiplier 416 with the signals (cos Wcmt), where wcm is the carrier frequency for the mth signal generator 302 of network 100. In the second path, multiplier 418 combines the signals from Walsh-code multipliers 412 with the signal PQ(t) and the signals from multiplier 418 are then combined by multiplier 420 with the signals (sin Wcmt). PI(t) and PQ(t) are the in-phase part and the quadrature-phase part, respectively, of short PN codes used in quadrature-phase shift-keying (QPSK) spread-spectrum modulation. As such, multipliers 414 and 418 may ensure that the signals are spread over the full carrier-frequency band. Multipliers 416 and 420 provide in-phase and quadrature modulation of the signals, respectively, by the carrier frequency wcm.
The signals from multipliers 416 and 420 are combined at summation node 422 to generate one of M low-power RF signals transmitted from each IS-95 signal generator 302 to combiner 304 of FIG. 3. Multipliers 414-420 and summation node 422 combine to operate as a signal modulator/spreader.
The 1.25-MHz bandwidth in the IS-95 standard limits the data rate with which a remote user can access the system, since the present IS-95 standard specifies low-rate data transmission for a single user. To achieve an even higher data rate for a user, one new proposed wideband CDMA standard defines CDMA processing occupying a 3.75-MHz bandwidth, rather than the 1.25-MHz bandwidth of IS-95. Allowing for guard bands at the edges of each 1.25-MHz IS-95 carrier-frequency band, the 3.75-MHz bandwidth of the wideband CDMA standard occupies a 5-MHz total bandwidth.
FIG. 5 illustrates the relationship between carrier frequency bands for the IS-95 standard and the proposed wideband standard occupying a 5-MHz total bandwidth. Each of three IS-95 carrier-modulated digital streams occupies respective 1.25-MHz carrier-frequency bands 503, 504, and 505 centered around respective carriers f1, f2, and f3. The 3.75-MHz wideband CDMA signal of carrier frequency band 502 occupies a 5-MHz total bandwidth spectrum, and is equivalent to that occupied by three IS-95 carrier-frequency bands.
Networks conforming to the IS-95 standard are limited to 64 users for each carrier frequency. Moreover, each user is limited to relatively low data-rate communications such as telephone-based voice signals. Under the IS-95 standard, each data stream is limited to a maximum of 9.6 kilo-bits per second (kbps) or 14.4 kbps. Thus, while IS-95 networks are sufficient for typical use by multiple mobile telephone users, they are nevertheless unable to support high data-rate applications. It is desirable, therefore, to design a wideband CDMA wireless communications system that supports high data-rate applications higher than those supported by conventional IS-95 networks. The transmit RF chain is generally one of the most expensive parts of a base station design, and it is desirable to reuse these components in an existing base station that is updated to handle both IS-95 and wideband CDMA communication. Since the equipment for such communication networks is extremely expensive and since an IS-95 infrastructure already exists, it is also desirable to provide a solution that is backwards compatible with IS-95 technology and the existing IS-95 infrastructure.
The present invention is directed to an RF transmit portion of a base station which supports, in a single RF processing portion or RF subsystem, either 1) low-rate CDMA communication channels alone, such as those confirming to an IS-95 standard; 2) high-rate CDMA communication channels alone, such as those conforming to proposed Wideband CDMA standards; 3) both types of communication channels together in a frequency overlay; or 4) combinations of these in different wideband carrier frequency bands, for example, 5-MHz bands. In accordance with the present invention, components of the RF subsystem may be shared between the low-rate and high-rate CDMA systems within the base station.
In accordance with the present invention, a transmitter of a CDMA network is adapted so as to overlay a frequency band of a high-rate CDMA channel onto frequency bands of one or more low-rate CDMA channel signals. The transmitter includes a high-rate CDMA processor, which generates two or more component CDMA data signals for each user data signal received by the high-rate data processor, and one or more low-rate CDMA processors, each generating a low-rate CDMA channel signal for at least one user data signal received by the low-rate CDMA processor. The transmitter further includes a combiner section, adapted to combine each component CDMA data signal with a different low-rate CDMA channel signal and a carrier signal to generate a low-power modulated carrier signal. For a further embodiment, an amplifier receives each low-power modulated carrier signal and generates a high-power transmit signal, wherein the power of the high-power transmit signal is greater than the power of each low-power modulated carrier signal.